1. Field of the Invention
The present invention relates to a CO2 recovery system and a method of removing a depleted material, which is produced in the CO2 recovery system, from a reclaimer in a short period.
2. Description of the Related Art
In recent years the greenhouse effect has been pointed out as one of causes of the global warming, and a countermeasure against it is urgently required internationally to protect global environment. CO2 emitted into the atmosphere has been considered the prime cause of the greenhouse effect. CO2 sources range various fields of human activities, including burning of fossil fuels, and demands to suppress their CO2 emission from these sources are on constant increase. Scientists have energetically studied means and methods for suppressing emission of CO2 from power generation facilities such as power generation stations which use an enormous amount of fossil fuels. One of the methods includes bringing combustion exhaust gas of boilers into contact with an amine-based CO2-absorbing solution. This method allows removal and recovery of CO2 from the combustion exhaust gas. Another method includes storing recovered CO2, i.e., not returning the recovered CO2 to the atmosphere.
Various methods are known to remove and recover CO2 from combustion exhaust gas using the CO2-absorbing solution. Japanese Patent Application Laid-Open No. H5-245339 discloses a method of contacting the combustion exhaust gas with the CO2-absorbing solution in an absorption tower, heating an absorbing solution having absorbed CO2 in a regeneration tower, and releasing CO2, regenerating the absorbing solution, and circulating the regenerated absorbing solution to the absorption tower again to be reused.
As shown in FIG. 3, in a conventional CO2 recovery system 1000, CO2-containing exhaust gas 1002 discharged from a factory 1001 is cooled with coolant water 1003 in a cooling tower 1004. The factory 1001 can be a boiler. The cooled CO2-containing exhaust gas 1002 is then conveyed to an absorption tower 1006 where it is brought into countercurrent contact with CO2-absorbing solution 1005. The CO2-absorbing solution 1005 can be an alkanolamine-based solution. CO2 in the CO2-containing exhaust gas 1002 is absorbed into the CO2-absorbing solution 1005, that is, CO2 is removed from the CO2-containing exhaust gas 1002. The CO2-absorbing solution 1005 containing CO2 (hereinafter, “rich solution 1007”) is conveyed to a regeneration tower 1008. The rich solution 1007 drips downward in the regeneration tower 1008. When the rich solution 1007 reaches a lower portion of the regeneration tower 1008, most of the CO2 absorbed ii the rich solution 1007 is released, and the rich solution 1007 turns into lean solution 1009 capable of working as the CO2-absorbing solution 1005. The lean solution 1009 is returned to the absorption tower 1006 and it is reused as the CO2-absorbing solution 1005.
In the CO2 recovery system 1000, material such as a sulfur oxide (SOx) remain un-removed in a desulphurization step. Such residual material reacts with alkanolamine contained in the CO2-absorbing solution 1005 in the CO2-removing process or an amine is oxidized in a process of recovering CO2 front the exhaust gas thereby producing a thermostable salt. The thermostable salt mixes with the lean solution 1009, which creates various issues. The thermostable salt cannot be removed under normal conditions in a course of producing the lean solution 1009 from the rich solution 1007, so that the thermostable salt accumulates in the system while the lean solution 1009 circulates.
The CO2 recovery system 1000 includes a reclaimer 1040 to which the lean solution 1009 that is produced in the regeneration tower 1008 is supplied. The reclaimer 1040 heats the lean solution 1009 to produce a condensed depleted material such as a salt. The condensed depleted material is then removed.
More particularly, the lean solution 1009 passing through a lean-solution supply line 1022 is extracted through an extracting line 1041 that is stretched from the lean-solution supply line 1022 to the reclaimer 1040, and the depleted material is removed from the extracted lean solution 1009 in the reclaimer 1040. The lean solution 1009 is heated in the reclaimer 1040 to, for example, 130° C. to 150° C., so that CO2-absorbing solution is vaporized from the lean solution 1009. A vaporized CO2-absorbing solution 1047 is supplied to the lower portion of the regeneration tower 1008. A condensed waste-product 1042 accumulated on a bottom of the reclaimer 1040 is removed, for example, by pumping with a pump.
Assuming that the capacity of the CO2 recovery system 1000 to recover CO2 is about 300 tons per day, about 10 m2 of the condensed waste-product 1042 accumulates in the reclaimer 1040. A cooling period of one to two weeks is required for the condensed waste-product 1042 to cool-down before it can be removed.
A reclaimer with higher capacity for accumulating condensed waste-product is required for a CO2 recovery system having a higher capacity to remove CO2. For example, if the capacity of the CO2 recovery system 1000 is to recover 1,000 tons to 3,000 tons of CO2 per day, 30 m3 to 100 m3 of the condensed waste-product 1042 accumulates in the reclaimer 1040.
In the large-capacity CO2 recovery system, which employs the natural-cooling method, a longer cool-down period is required. It may take one month or longer or the cooling-down period in a near feature. There is a need for shortening the period for which the condensed waste-product stays in the reclaimer.
In addition, in the case of the natural-cooling method, the depleted material that is being cooled-down adheres to a steam supply line 1050 for supplying saturated steam 1046, so that the steam supply line 1050 needs to be periodically detached for cleaning.